Selective catalytic reduction of nitrogen oxides by ammonia on iron oxide catalysts

In this study, new Fe2O3 based materials are developed for the selective catalytic reduction (SCR) of NOx by NH3 in diesel exhaust. As a result of the catalyst screening, performed in a synthetic model exhaust, ZrO2 is considered to be the most effective carrier for Fe2O3. The modification of the Fe2O3/ZrO2 system with tungsten leads to drastic increase of SCR performance as well as pronounced thermal stability. These results show that tungsten acts as bifunctional component. The highest catalytic activity is observed for ZrO2 that is coated with 1.4 mol% Fe2O3 and 7.0 mol% WO3 (1.4Fe/7.0W/Zr). By the use of this catalyst quantitative conversion of NOx is obtained between 285 and 430 °C with selective formation of N2. Here, the turnover frequency of NOx per Fe atom is found to be 35 × 10−5 s−1 that indicates a high catalytic performance. The SCR activity of the 1.4Fe/7.0W/Zr material is decreased in the presence of H2O and CO2, whereas it is increased by NO2.Temperature programmed reduction by H2 (HTPR) analyses show that the Fe sites of the 1.4Fe/7.0W/Zr catalyst are mainly in the form of crystalline Fe2O3, whereby relatively small oxide entities are also present. The strongly aggregated Fe2O3 species are associated with the presence of the promoter tungsten. Based upon stationary catalytic examinations as well as diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) studies we postulate an Eley Rideal type mechanism for SCR on 1.4Fe/7.0W/Zr catalyst. The mechanistic model includes a redox cycle of the active Fe sites. As first reaction step, we assume dissociative adsorption of NH3 that leads to partial reduction of the iron as well as to production of very reactive amide surface species. These amide intermediates are supposed to react with gaseous NO to form N2 and H2O. In the final step, the reduced Fe sites be regenerated by oxidation with O2. As a side reaction of SCR, imide species, originated from decomposition of amide, are oxidized by NO2 or O2 into NO.